This study examines the impact of CLIL on the English language competence achieved by students in primary and secondary education in the monolingual autonomous community of Extremadura (Spain) and extends investigation to a year after the end of the secondary CLIL programme when students were finishing their first year of "bachillerato." To compare CLIL and non-CLIL strands effectively, the homogeneity of participants was carefully controlled. This study also looked at 'type of school' (public schools vs. charter schools) as a variable having a potential differential effect on students' L2 achievement. Comparing CLIL and non-CLIL groups in public schools, significant advantages were observed for CLIL pupils at the end of secondary education and were even more marked at the end of the first year of "bachillerato." However, when comparing public school CLIL students with charter school non-CLIL students, no statistically significant differences were found in the language competences tested. The public school non-CLIL students showed significantly lower performance on all measures at all levels compared with charter school non-CLIL students. Thus it could be argued that CLIL streams in public schools may help to compensate for the advantage that charter schools typically represent in general academic achievement and English language competence.